Regular cast:
Doctor Who (Tom Baker)
Leela (Louise Jameson)
Voice of K9 (John Leeson) on The Invisible Enemy and from The Sun Makers Part One onwards

Regular crew:
Producer: Graham Williams.
Script Editor: Robert Holmes until The Sun Makers Part Four. Anthony Read from Underworld Part One onwards.
(Anthony Read also contributed to the script editing of Image of the Fendahl and The Sun Makers but was uncredited on screen.)
Production Unit Manager: John Nathan-Turner.
Title Music: Ron Grainer and the BBC Radiophonic Workshop, arranged by Delia Derbyshire.
Special Sounds: Dick Mills except on The Sun Makers. Paddy Kingsland on The Sun Makers.

Graham Williams encountered problems almost as soon as he arrived as Doctor Who's producer. The intention had been that season fifteen would open with a Terrance Dicks vampire story entitled The Witch Lords (also referred to as The Vampire Mutation), but this was vetoed at a late stage by Head of Serials Graeme McDonald on the grounds that it could be interpreted as a send-up of a lavish BBC production of Bram Stoker's Dracula that was then in preparation.

Consequently, the production team - on which Robert Holmes was shortly to be succeeded as script editor by a highly experienced writer/producer named Anthony Read - had to find a hasty replacement and rearrange the season's production order so as to allow time for it to be written and recorded. The first story to be made was consequently the second to be transmitted. This was The Invisible Enemy, which saw the introduction of a rather unusual new companion to join the Doctor and Leela on their travels.

The robot dog K9 had been envisaged by writers Bob Baker and Dave Martin as simply a one-off character for their story, but Williams decided that it was too good a creation to waste on just a single appearance. He saw it as a potential ratings winner, particularly amongst children, and hoped that it would help to boost the level of humour in the series - something he was keen to do as he was under considerable pressure from BBC management to tone down the horror content that had caused such controversy during his predecessor's tenure. Tom Baker, by contrast, was less than happy about the dog's introduction, believing that the Doctor really had no need of any companions at all. Fortunately, he did get on well with John Leeson, the actor chosen to provide its voice.

The replacement first story, meanwhile, was another Terrance Dicks contribution that would see the Doctor and Leela battling for their lives in a uniquely isolated historical setting...

To add to all the many problems that Graham Williams had faced during his first year on Doctor Who, Leela had to be written out in The Invasion of Time after Louise Jameson decided against renewing her contract. Consequently a new companion character would have to be devised and cast in time for the next production block.

The Invasion of Time had seen perhaps the greatest shift yet toward the more overtly humorous style that Williams was attempting to cultivate. There were indeed some within the BBC who considered that this had gone too far, and it was agreed that all the series' directors should in future receive a memo reminding them of the need to preserve the essentially serious nature of the stories. It was also agreed that Tom Baker's frequent and often comedic scripting suggestions and ad libs should be kept more closely in check during the course of the following season - a season that would adopt a format unprecedented in Doctor Who's history.